Cosubstitution of Ca 2+ for Y 3+ and La 3+ for Ba 2+ in Y 1- zCa zBa 2-2 yLa 2 yCu 3O 6+ x has demonstrated that, in this system, there is a critical oxygen concentration xc=0.96 + 0.5 nc, where n=2 y- z, such that disordering of the oxygen in the Cu(1)O x planes has no effect on Tc for x< xc, but creates mobile-hole traps for x> xc. Morever, substitution of La 3+ for Ba 2+ in the Ba 1- yLa yO layers creates local disorder of the oxygen neighboring a La 3+ ion, which reduces the orthorhombicity of the structure. These two observations explain why the systems Y 1- zCa zBa 2- zLa zCu 3O 6+ x and LaBa 2-2 yCa 2 yCu 3O 6+ x=La 1- zCa zBa 2- zLa zCu 3O 6+ x maintain a Tc > 80 ( ) K through an orthorhombic-tetragonal transition with increasing z whereas LaBa 2-2 ySr 2 yCu 3O 6+ x=La 1- zSr z(Ba 2-2 ySr 2 y- zLa z)Cu 3O 6+ x exhibits a Tc that decreases monotonically with inc reasing y. They demonstrate that Tc is independent of the space-group symmetry where hole trapping does not occur. Decreasing the lattice a-parameter and/or the basic character of the BaO basal “planes” degrades Tc in a relatively minor manner. These findings also implicate c-axis oxygen in any oxygen disordering of the BaO-Cu(1)O x-BaO intergrowth layer.